Experiencing the same, after an upgrade to 2.3.13. In my case, all attributes are still visible (attached to products) in the admin, apart from one specific attribute (“grape/druivensoort” – it’s a wine dealer). Saving the product then removes another attribute from the product (?) but further saving makes no more strange changes.
All the details seem to be still present if I perform a product export, and no amount of adding attributes etc. seems to trigger re-syncing of any kind.
Any advice welcome!