Line spacing settings ignored

In Writer, any settings using “Fixed” below 0.20" or “Proportional” below 100% are ignored completely, and the “Decrease line spacing” toolbar button is greyed out. I am working within a table within the document which just barely flows onto a second page because of the unchangeable broad line spacing of its text.

Had been using v.5.3, but upgrading to 6.1.3.2 just now on the assumption that it might be relevant did not resolve the issue. (Asus GL553VD i7-7700HQ @2.8ghZ Win10 (v.1803 build 17134.472))