I've been in that arena a few times. Basically, the unburnt powder is telling you that your load is not "in the zone".
Interestingly, in my Sask commemorative, I did use just about the same load, a cast bullet, a LEE 379-250, and 20.7 gr of H4198, and various LR primers. I do not recall any unburnt powder issues. I did prefer some other powders, like SR4759, 5744 and Reloader 7, but H4198 was never an issue. The 38-55 and 375 are similar, the 20 gr charge is in the book, IIRC. Although similar in burn rate, IMR and H 4198 have somewhat different characteristics. You referenced AR2207, same as H4198.
I've read that most handloaders shooting the 375 Winchester use 38-55 cases and data for shooting cast, there may be something in that. With the 38-55, the groove diameters are all over the map, and usually bullets need to be 0.379" to work. Have you measured your groove diameter?
4 or 5 granules (unburnt kernels) is not much, with 4227, in a 44-40, I collected enough to put in a small pile and was able to light it with a match, totally unburnt kernels were left in the case. If you chrono your load, you will be able to tell if you are having ignition issues.
To illustrate, here are the chrono results from that 44-40 load. Notice the difference powder position makes.
Velocity Av 664/860 Es 80.5/59 Sd 35.3/25.23 Shots 5>/5<
Nitro