If you have established drops prior to swapping scopes, that should hopefully trim down error potentials. However, your previous drop doesn't sound right. Zero'd at 100 yards, 1.5" (~0.2 mil or ~3/4 MOA) at 200 yards, 7" (~0.7 mil or ~2 1/4 MOA) at 300 yards, and 12" (~0.8 mil or ~2 3/4 MOA) at 420 yards? There's 100% something wrong with that data set.
With your previous set-up using hold-overs, did you verify that the reticle subtended correctly? If you're using SFP, did you calibrate to see what magnification setting you needed to be on? It also becomes difficult to establish good drop data using holdovers unless you have a specific hold that actually lines-up dead nuts exact (pretty rare). Also, it is extremely rare for anyone to have an exact 100-yard zero - you almost certainly have a zero offset.
I asked about distance to targets because I've seen established ranges be about 5 - 10% off from what they think their ranges are. Unless you verify with a known rangefinder/tape/etc., you may be gathering bad data. Obviously, even if the ranges are incorrect, you can compare the previous drops to what you're currently experiencing (but again, there's an error in your previous data).
Assuming your four 4-round groups maintained about 1.5-inch aggregate group in the same location on target, you may not have an issue with fasteners being loose or a scope issue. If the 'groups' are floating all over, you might check torque on everything as well as barrel/bolt/etc. to see if there is unwanted contact anywhere, and you might look harder at the scope to see if there's an error.
Although not stated, I assume you are holding over with your new optic? Have you checked to see if the reticle subtended correctly? Have you calibrated to find the correct magnification that the reticle in that Zeiss should subtend?
Obviously not knowing you or your set-up it could be a handful of things. Right now, based on that previous data set and what you're currently experiencing, I'm wondering if it isn't an error in how you are using your optics to hold over. You could dial the optic to take out the potential of error that could arise from holding over. However, you should also check that your optic actually tracks correctly before doing that. So, you might consider setting-up tall targets at 200 and 300 yards and aiming exactly at center of target and then measure the drop. I'd guess you should be about 3-inches(ish) low at 200 yards and about 12-inches(ish) low at 300 yards. From there, confirm that you are using the reticle correctly, and that the reticle also subtends what you think it should.
