Anything more specific? Screenshots, numbers, etc.
Have you changed a difficulty? Which level are you talking about? "All of a sudden" means after what?

My apologies. I will try to be more specific.

Recently (23rd? 24th? Date unsure) I was able to unlock "Prestige ticket/bit dust drop rate", the second "crown" on the first tier of prestige upgrades. Screenshot attached to show level.

I think it was this massive boost which took me to getting about 100 bit dust per long level (using most abilities). Screenshot attached showing both a long level and short level result. These show how the bit dust being received is about 50% less.

I cannot give a Screenshot of the 100 bit dust result, it's not something you take pictures of! I don't think I had any temporary boosts on which would have given me a few levels of double bit dust.

When it stopped? The 26th, when I raised the ticket.

That research on the prestige branch affects only prestige tickets / prestige dust.
Quests in Endless mode give bit dust too, it may be the case - quest rewards and regular loot stack up on the "game over" item list and it is possible that you've completed some quest which gave more dust than during your other run.
Please check the "received items" list next time, it should show separate lists of items for regular loot and quests. If you encounter this issue again and the list will show significantly lower amount of dust received for two almost identical "Game on level X.X" please report them here.
Currently I can't do any math based on your screenshots to tell if there's actually any issue. You may also want to discuss this issue on Discord, there are players who do amazing job on analyzing such things and chances are, they know how this thing works better than me

That's great, thank you very much! I think I've missed a "bonus" dust award or loot on the level and saw only the headline amounts. Seems to be relatively stable now. Still weird though but I'll definitely follow your advice.

Ps I'd close the thread for you if I could!