Yes, this issue appeared somewhere after 7.20, when message field became multiline.
I've reported about this issue and they explicitly refused to fix it and recommended to use v4. Moreover, v4 had "beta" label at that time...
I will try to fix it in my custom ESR version, however I can't promise anything for now.
UPD: I've fixed the issue in my custom ESR version, the fix will be included in the upcoming version 3. Will be released within a week or two.
The topic is here: WinBox v3.43 ESR (bugfixes, improvements)