Hi!
I have the same problem.
An extra category is randomly added to /manufacturer/name/ when I click on manufactuter name on product details page.
The normal manufacturer category should look like this :
https://www.kovrovik.com.ua/manufacturer/confetti-carpetBut instead I have this:
https://www.kovrovik.com.ua/manufacturer/confetti-carpet/kovryI dont see any regularity. It can be the last visited category, or the canonical category or anything else.
This completely changes meta tags of the page and shown products: on some categories there is only category description and products, on others there is nothing.
Joomla cache turn off doesnt help.
The only thing that seems to help is turning off Joomla SEF as well as Virtuemart SEF.
Upd. I noticed that this is most likely Virtuemart SEF because the added ending is from Virtuemart category alias not from Joomla menu.
Virtuemart 3.4.2.
Joomla 3.9.8.
Php 7.2.21.