When research gets too great, techs no longer complete researching. Instead, the tech bar fills up and resets to empty at the end of a turn and the tech never finishes. I created this bug with a very large empire on insane size with every colony building Research Project. I was able to get the researched tech to complete by switching production on 10 or so colonies to focus on economy instead of research (that is, lowering the amount of research generated per turn allowed the tech to finish).
Note that this is in a modded game. It may not be possible to recreate this situation on a vanilla game. However, there does appear to be an "overflow bug" in the research calculation.