Yeah, this is a tough one. People don't use MG usually because of the added nutes. You have no idea what's in it at any specific time other than when it's sitting in an unopened bag. Once you add water you don't know how much of which nute has been released. I know the bag says that it feeds plants
'Up to six months' but that could mean that the soil could feed your plants for three months one time, one month the next, and six months the time after that. There's really no way to tell unless you're an expert at diagnosing weed, even then it's difficult doing it to someone else's plant through a pic.
suTraGrow seems to be fairly confident on the phosphorus def and your plant even shows other symptoms associated with that, but it could be deficient due to lockout caused from wacked out PH from any remaining MG nutes and the added nutes. Adding more nutes would only make it worse.
I would flush the fuck out of that plant since it couldn't hurt, and when you think you're done, flush some more. Check the PH of the runoff when you're done, once it's 6.3-6.9 you're good. By then you shouldn't have to worry about any of the MG nutes either, they should be gone. Here's a chart to show that with the combination of any remaining MG nutes and the nutes that you added it could easily lower your PH to below 6.0 locking out phosphorus.
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