I've been running seti@home on my two "production" Windows machines for about a week to see if I can make use of the idle time without getting in the way when I need to use one. So, I've been watching the process way too closely and tweaking things way too much.
I was surprised a few days ago to see a task with an estimated completion time of over 30 hours, so I watched that one even more closely. It eventually finished, apparently normally, in about 24 hours.
But, when I look at the task list on the Web site, there's no trace of it.
It was an AstroPulse task, so it should have been pretty easy to find.
The machine ID is 8891809, I5-9400-based, running Windows 10 Pro.

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When the secondary database server isn't playing hide and seek with up to one day running behind on the primary server, in general it takes just 24 hours for a task to be visible on the server once it got uploaded, reported, checked against its wing person's outcome and validated. So it can easily be that your wingmate had it reported earlier and by the time you looked in, it had validated and been deleted from the lists already. It's impossible for the project to keep all tasks forever on display, because at the moment they're already in uncharted territory for database sizes.

That said, do make sure you were watching the right system.
Your ID: 8889124 shows an AP that took close to 10 hours, whereas your ID: 8891814 shows one AP that took 5 hours+.

Can tasks just as that disappear?
Well, maybe if they were timed out already. Then their result was removed from the server already by the time you uploaded & reported it.

Timing out makes sense, but I'd hoped to see what 24 hours of compute time was worth, credit wise.

As for the other two machines, 8889124 is an old Linux machine that's been sitting in a corner of the basement doing practically nothing but einstein@home for some time (5 million credits worth), and 8891814 is the other Windows 10 "production" machine that again spends most of its time doing nothing. I use the term "production" very loosely. One is the household PC for any and all tasks and the other runs my ham radio station.

I had thought about running einstein@home on the two Windows 10 machines too, but decided to switch all three to seti@home, which is what I started with several years ago.

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Timing out makes sense, but I'd hoped to see what 24 hours of compute time was worth, credit wise.

Credit isn't based on time, but on the amount of floating point operations the task took. This means that a slow CPU that takes several days to complete such a task will get as much credit as a fast CPU that took minutes.

Astropulse tasks take in the region of 400+ credit per task.

Just noticed that it happened again. From the Boinc log:

2020-02-09 20:58:05 | SETI@home | Starting task ap_28ja20ad_B3_P0_00348_20200208_30535.wu_1
2020-02-10 01:51:30 | SETI@home | Computation for task ap_28ja20ad_B3_P0_00348_20200208_30535.wu_1 finished
2020-02-10 01:51:32 | SETI@home | Started upload of ap_28ja20ad_B3_P0_00348_20200208_30535.wu_1_r778400261_0
2020-02-10 01:51:34 | SETI@home | Finished upload of ap_28ja20ad_B3_P0_00348_20200208_30535.wu_1_r778400261_0

2020-02-10 03:26:23 | SETI@home | Computation for task ap_05fe20aa_B6_P1_00103_20200206_19980.wu_1 finished
2020-02-10 03:26:25 | SETI@home | Started upload of ap_05fe20aa_B6_P1_00103_20200206_19980.wu_1_r853965324_0
2020-02-10 03:26:28 | SETI@home | Finished upload of ap_05fe20aa_B6_P1_00103_20200206_19980.wu_1_r853965324_0

The start for ap_05fe20aa_B6_P1_00103_20200206_19980.wu_1 apparently ran off the top end of the log.

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There is a limit on the number of lines available in the log, and once that limit is reached then the oldest lines are discarded and the newest inserted. It is possible to increase that limit, but I can't remember where it is stored.

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Bob Smith
Member of Seti PIPPS (Pluto is a Planet Protest Society)
Somewhere in the (un)known Universe?

where it is stored.

Documented here and is called <max_event_log_lines>N</max_event_log_lines>