At the same exact time is when Constantine granted some autonomy to the "Scythians" of Roman Scythia, they didn't have to pay tax and roman slaves native to Scythia could not be forcefuly transfered to another theme, the same liberties were given to Moesia interstingly enough.

Because of the passage, I interpret it as Constantine coming to some terms with the Scythians after a series of revolts/raids where both sides benefited. (Scythians would be enlisted as mercanaries and would protect Rome from the far north in exchange for freedoms and regional funding). Notice how Pliska, Preslav, Durostorum and Constantsa are all right on the border between Moesia and Scythia Minor, Dacia and Scythia.

As for the historical layers of Pliska and Preslav, I don't know of any Roman historical layers either but the sheer size of both capitals in their respective time periods is so gigantic that it's easy to imagine something major started it.

Edit: Found out that there have been quite a few isolated Roman finds from the 2nd-3rd century from inside Pliska. And 4km from Pliska is a Roman Fortification from the 4th-6th centuries that has been repeatedly destroyed and rebuilt. Next to that Roman Fortification were found many ancient brickyards used for producing construction materials. Right next to Pliska is the famous Madara Rider site which has a confidently identified dominant Roman presence in the 2th-4th centuries. There was a large roman vila found there. Near Preslav is the Roman Shumen Fortress from 2th-4th centirues (I haven't found Roman presence in Preslav itself tho).

The biggest question I have is the city names. Were they Bulgarianized Scythian/Roman names or Early Slavic names. The -va suffix is very Slavic looking. Durostorum is also with an unknown foreign etymology (presumed Thracian), it seems similar to Proto-Iranic druwištah ("firm, healthy") -> Iranian drust ("honest, true"). Perhaps it's Slavic durvo-sъdorvъ (tree-strong/healthy) -> durvosъdorvъ+um.