I found out that RS_MText does not support text attributes from other CAD systems. Specifically, \W, \H and \Q. Non-working example can be found at https://github.com/LibreCAD/LibreCAD/issues/850 as attachment. Those attributes appear as a part of original text and that is incorrect for sure.
QCAD seems to support RTF text rendering with these attributes and several other ones. It has proper infrastructure for complex text handling, bounding box changes et cetera.
Does anyone have some kind of ready plan of implementing rich text in LibreCAD? Maybe it is irrelevant now, because LibreCAD 3 is on the way, but it happens I have some time to maybe fix this.
I've implemented simple skipping of those tags but I don't think it is of any use for the project.