Thanks everyone. Platen gap was the problem in my case. Paper feed also had en effect, but simply setting the platen to "standard" appears to work just fine. The platen is then adjusted accordingly to media type selected (which works in my case) as mentioned here:
https://imageprinttoday.wordpress.com/2011/07/14/the-platen-gap/I had "widest" selected, which *should* be the appropriate setting for ~300gsm paper, so having issues under the supposed correct setting has me a little confused...
The other thing that's still throwing me off is, I have noticed that prints from a previous roll of Epson Hot Press apparently have this problem also, now that I inspect them more thoroughly, though barely visible, and with this new roll of Epson HPB, under the same settings, the problem is really blatant... oh well, all seems to be ok now.