Villagers Glitch when they are on their workstation and there are a trapdoor over their heads

Hi Everyone, 

I build a machine to put my villagers on a sales stand. However, I don't know how and why but when they works on his workstation ( Lectern, smoker, etc. ) They changes job constantly and it's very fast. ( it's annoying when you have a villager with Mending XD )

Because I don't want that my villagers leave the stand, I puted a trapdoor over the stand for my villagers can reach his work job.

To resolve problem, I have to close minecraft (Come back in the menu is enough) to resolve the problem temporaly.

If the problem is just me, How I can resolve ?

However, if is a real bug, thanks to all developer who work hard to make this game greatfull.

I share you a short video to show you how blocks are placed and the realtime reaction.

 EDIT 1 : I think I have resolved the problem, I change the workstation place. If you look the GIF, I place it where the slab between two log. but it's even a bug according to me.

PS: Sorry if you find some mistakes, it's not my first langage.

Thanks you !