I have a villager trading hall with over 50 villagers.  The trades have been very stable, with the exception of a recent glitch.  

I added a new fletcher and a new librarian at the same time (the librarian was not involved, but I am noting it for context), I was looking for a new tipped arrow trade from the fletcher.  When the fletcher was levelled up (I had not touched the librarian) he traded an "arrow of healing".  I traded for 5 arrows and put them in a chest, I immediately tried the trade again and his tipped arrow trade had changed to "arrow of swiftness" - a trade I already had.  

I have 3 other fletchers trading for tipped arrows and all their trades are now for "arrow of swiftness".  With the exception of the fletcher that had a trade for "Arrow of swiftness", they each had a different tipped arrow trade prior to this issue - each was labelled with a sign, so I am certain they changed.  I have two other fletchers trading for an enchanted bow or enchanted cross bow, neither of their trades were affected.

I should note that my trading hall is built with a zombie spawner at the centre to take advantage of reductions in trade cost when a Zombie villager is killed.  I think this is taking advantage of another bug, but it means ALL my villagers are offering reduced cost trades, including the new fletcher and all the other fletchers.

None of the other villagers trades changed.  It was only the Master Level tipped arrow trade from the fletchers.

All of this is on an iPhone 8 with the world hosted on a server.