Hi,
I cannot confirm this under the following conditions:
Tools -> Properties for All Configurations -> Highlight (1)
☑ Enable Keyword Highlight
Only User-Defined Strings, e.g. highlight
Color: Highlight (1)
nothing else ticked off.
What are your settings?
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Windows 10 Pro (x64) Version 2004 (OS build 19041.329)
EmEditor Professional (x64) v19.9.3 and v19.9.901 (portable)