After 6.2.6 release of Magnolia, the issue seems to affect very specific setups, with the following confirmed (add more if encountered elsewhere), revalidated with Magnolia DX Core Version (bundle) 6.2.7 (Snapshot: 2021.03.15 11:20:56):

Setup 1:
(Google Chrome Version 88.0.4324.150 (Official Build) (64-bit), Firefox 85.0 (64-bit), Linux Mint MATE 19.3, OpenJDK Runtime Environment (build 11.0.9.1+1-Ubuntu-0ubuntu1.18.04))

1. Using Magnolia CLI, install DX Core empty web app.
2. Add the bookshelf light module to the author instance (git clone https://git.magnolia-cms.com/scm/documentation/bookshelf.git).
3. Open the Author catalog app and add a new item:
   use test item as the name, and add a single tag. 
4. After saving the edit dialog, the browser shows this:
   (Google Chrome Version 88.0.4324.150)
   
   (Firefox 85.0)