I have to confirm this issue after more intensive testing through several past days and it is really a nightmare. 
It does not matter which Tvheadend backend is used, the same issue happens on both 4.2 and 4.3 Alpha, quite often during seek operations but randomly. The same issue on 2 different RPi 4B devices (2 GB, 1 GB), latest several LE 10 Nightly builds (currently 20210321).
So far I was not able to reproduce it with any specific tasks that I could describe but it seems to be related to this common error message in the log:
LibreELEC tvheadend[1677]: htsp: 127.0.0.1 [ | Kodi Media Center ]: Disconnected
In the same second it's reconnected again but the Timeshift data are lost.
If I am not mistaken, this means the Tvheadend client (8.2.3.1) is losing the connection to Tvheadend backend so the itself Client could be the reason of the issue? I have tried to increase the Connection / Response timeouts from defaults 10 / 5s to 20 / 15s but it did not help.