Suffering Acer(s)

Hello again,

Last week saw me pot up 3 Acers that I bought over winter. I have a Crimson Queen, Osakazuki, and another as yet unidentified upright variety that I rescued from the half price sale section at the garden centre (I figured it would be fun to see if I could bring it to life and see what kind it was). All were potted up one size.

I potted in 3 parts John Innes number 2 with 1 part horticultural sand, compacted the potting mix a little so it didn't sink too much. I used terracotta crocks and stoney gravel in the bottom of the pots which have adequate drainage holes.  I then sprinkled a little slow release fertiliser in and topped with forest green chippings. I made sure the trees were planted up to the same level as they were in the pots I bought them in. Once done I gave them a good water just until it started to run out the bottom of the pots which it did within about 20-30 seconds.

I have noticed today that the bark around the graft union of the Crimson Queen is starting to flake off and the trunk underneath seems unhealthy, almost as if it is rotting. That said the branches are starting to bud out with what look like healthy leaf buds and the branches themselves seem fine.

I have made sure none of the stones are touching the trunk and the soil underneath seems damp, but not soaking. Can the bark sometimes start to flake around graft unions like this, or should I be concerned? I did a lot of research on forest green chippings but couldn't find an answer as to whether they might be acidic/alkaline. 

Thanks in advance for any advice.