They banned importation of marking rounds, UTM and Simunitions FX for purely political reasons.
In particular, there is an anti-gun lawyer political appointee in Imports Branch who thought that marking rounds could be used as an anti-personnel round and therefore had no "sporting purpose" and could be banned from importation.
While I don't like it, from a legal standpoint, the government can do all sorts of import-related shenanigans if they feel like it with little legal recourse. The companies can only do internal appeals to ATF, which go nowhere.
Blanks can still be imported, because they have "sporting purposes."
UTM and FX can now be imported again, but only to government entities who do end-user certificates, and the ammo has to ship directly to them. There are no further restrictions though, and at some point in time, gray market ammo will pop up, as it will not be unlawful to possess it. But it will probably take awhile and there won't be reliable supply chains.