As in the title, disabling poly-org inside of a src block disables all fontification in the buffer.
I can recreate the issue, with a minimal init file, by opening an org file, placing point inside of an org src block (so that the active buffer is buff.org[src-mode]). A call to poly-org-mode disables the minor mode, but leaves the value of fontification-functions as nil (rather than jit-lock-function as might be expected).
In contrast, if I disable poly-org-mode while point is in the base buffer there is no issue.