Yeah, it seems that interlaced resolutions aren't supported at all in Navi. Been trying to get them on my 5700xt to no avail.
I'll send a bug report as well.
Same on an old Panasonic Viera HD ready. On previous gpu 4870 and 7870 it always worked fine in 1920x1080 at 50Hz. Since i've a 5700xt i can only have up to 720p. 1080i 50Hz isn't in the list of supported formats :/.
use CRU to set 1080i Resolution - you need the correct settings for that and enable the "interlaced" checkbox 😉