Turns out a Storm Troop- er, I mean
Millitarum Tempestus regiment is most definitely an Alpha Legion proxy that was so successful at infiltrating the Imperium that the Ultramarines not only love them as allies, but that they also draw recruits from the scions. Likely meaning that a fair amount of Ultramarines are sleeper agents. Which actually provides a handy excuse for a lot of the dumb material being produced for the Ultramarines by the Black Library, especially McNeil's Ultramarines series with the idiocy of Uriel and the rest of his Chapter. To some degree the Ultramarines have been comprised by either sleeper agents, or at indeed influenced by an Alpha Legion proxy. Which also goes hand with how the Alpha Legion has been preying upon Ultramarine descendents for a while,
IIRC one Alpha Legionnaire spent what was likely a century or more setting up an entire chapter to fail by indoctrinating the recruitment population via psyker mindfething with help from a Black Legion Warband. He then executed the fall of the Chapter in one swift movement that utterly annihilated a thousand astartes in one fell swoop. This could very well be the work of that same individual, building up a Tempestus Regiment to prepare for either heavily wounding the combat capability of the Ultramarines or possibly even crippling them. Or maybe trying to taint them to the path of Chaos- who knows. What we do know however is that on some degree the Ultramarines have been infiltrated.