To reproduce the bug first bug. I call it "twin group mutation":

Create a Drafting View.
Draw a few detail lines. Group them. Name the group "My detail group".
Draw another detail line (the mirror axis).
Mirror "My detail group" to the mirror axis.
Add a text annotation. Group it. Name the group "My text".
Open the initial "My detail group", click Add + and add the "My text group to the group.
Click Finish.

Watch as the mirrored group seemingly doesn't update. Select it move it to another location. Note that is does in fact contain the text but in its (un)mirrored form. You have two groups which should be identical but are apparently different.

To reproduce the bug second bug. I call it: "The instance property is alive!"

Create a Drafting View.
Draw a few detail lines.
Add a text annotation then copy a carefully formatted text in this annotation, one with tabs and multiple paragraphs. Adjust the size of the text box to suit the text. Juuuust perfect.
Group the lines and text note together. Name the group "My carefully designed detail group".
Draw another detail line (the mirror axis).
Mirror "My carefully designed detail group" to the mirror axis.

Observe the two text notes. The first is aligned to the right and the second is aligned to the left. This might seem right because I mirrored the group.
How can this be, the alignment is an instance property and I did not modify it. The text box itself has mirrored but Revit decided the text alignment should modify too. Apparently I can have two identical groups with members that are not identical in their Instance properties? How would the mirror command even know to modify the text alignment property?!

There is no way to have both texts entirely readable with formatting (even the tiny bit of formatting that Revit has). When one looks all right the other is wrong and the other way around.

One might say that the text rectangle has in fact mirrored and it has noted this shift in the left and right side and modified its own instance property to reflect the change. So it might be getting its left and right form somewhere else, like the page itself. While this might be the case, there is clear evidence that the text should maintain its own formatting and be "watchable" as well as "readable" with its notions of tabs and paragraphs. Also Autodesk gives the user no choice in the matter as it is clearly impossible to obtain the other behavior, you are locked in to Autodesk interpretation of "the right choice".