airwell highwall frosting

I have 're gassed an airwell high wall unit, don't have a model number but it's a small old r22 system and I have just swapped it over to r417a. I charged it to 85% of the r22 plate charge and for some reason there is a strip of frost right across the evaporator coil only in the middle about 2 inches in height. I tried taking the charge a tad higher to 90% of the original charge but made no difference, I've switched it off to thaw out then run again but still appears.

I have set the temperature to 20 degrees from 16 as I know from working on some Gree units when they're set that low they do the same thing. Has anyone had same issue with this type of unit? There is a red led which flashes on the front cover but can't find any info anywhere about what it stands for. I feel the unit is grossly under sized for the room but is something I will have to look into further if it keeps frosting, when set for 16 it wasn't knocking off but when raised to 20 it was and so stopping the frosting. I have worked on the odd airwell before and personally think they are a bag of sh!*e , was just wondering if anyone has had same issues?