In GNU, Heirloom, and Plan 9 roff, literal tab characters are

In GNU, Heirloom, and Plan 9 roff, literal tab characters are
non-breakable in exactly the same way as "\ ".  That is, the preceding
word, the tab character, and the following word are always kept
together on the same output line.  If filling is enabled and an
output line break is required before the end of the following word,
the break occurs before the beginning of the preceding word.

Make mandoc behave in the same way.

Of course, using literal tab characters in filled text remains a
bad idea, and the "WARNING: tab in filled text" remains unchanged.