I noticed that when during the game, I change research allocation from all techs to one, techs which do no longer have research points allocated to them do complete when they were close to completion before the allocation was changed.
Is this a bug or is there a hidden build up of research points which continues after research is stopped (similar to intel points) ?

it's a feature. It should represent a sudden unforeseen breakthrough, some kind of genius-like shortcut/cutoff leading to scientific breakthrough without further funding. Or let's say the private sector funded the rest because the new tech was close to completion (it's above 80% I believe, could also be 75%). Internally, dices are rolled each turn accordingly if such an event happens. You can use this of course to stop allocating further research points after a tech has hit the 80% level of completion.