I'm having the problem that some completely downloaded files are broken. When I try to play them they don't play correctly. I then force recheck them in Tixati and it turns out a bunch of parts are "broken" and have to be redownloaded.
See https://i.imgur.com/JRPQIe3.png
I'm using v2.88 portable but the problem was there in previous versions. They did complete downloading w/o trouble and are shown as standby.
Maybe someone else is having similar problems?
I don't want to reset the whole client and loose all torrents.

I've not had this issue, torrents always hash check fine.

I hope for your sake it's not, but it's entirely possible your storage is failing,

Check with Crystal Disk Info that your HDD isn't breaking down. Tixati would refuse to save any pieces that can't pass integrity verification.

I have seen this, too. I download 2 versions of a video and one breaks up and the other is fine. I even do a force check on the bad file and no problems are reported.

OP here.
I checked my HDD with CrystalDiskInfo and everything looks fine. No errors.

Weird if you are to assume there's no HDD/OS fault then either the video file is broken (WaltP's case) or Tixati is...
OP, can you download the same file with qBittorrent? If you have 7-Zip installed then right click on both files downloaded through Tixati and QB then > "7-Zip Hash" > Choose any name from the list, like MD5. If the 0123abcdef values match for both files then they're absolutely equal. If they mismatch, one of the clients downloaded garbage.

PS: There're other ways to get a file's hash but its the simplest way to.

I have the same issue and never figured it out.
I just force check completed downloads and occasionally some will revert to having missing pieces. Usually they're 96-99% intact, but the worst case I ever had was a completed file going to 87% upon re-check.
No errors on my hard drive. I have a dedicated one for torrents.