I could reproduce under both trunk and 4.08.

Under 4.08, the test log in _ocamltest/tests/tool-debugger/find-artifacts/debuggee/ocamlc.byte/debuggee.byte.output says:

Loading program... done.
Breakpoint: 1
10   <|b|>print x;
Debugger [version 4.08.0+dev6-2019-03-01] environment error:
 The files /usr/local/lib/ocaml/stdlib.cmi
 and /home/gasche/Prog/ocaml/github-4.08/testsuite/tests/tool-debugger/find-artifacts/_ocamltest/tests/tool-debugger/find-artifacts/debuggee/ocamlc.byte/out/blah.cmi
 make inconsistent assumptions over interface Stdlib

which suggests that the problem come from a discrepancy between the test environment and a /usr/local OCaml install on my system -- the test environment should be configured so that this local install is never used by the testsuite.

Interestingly, the failure log on trunk is much less readable:

Loading program... done.
Breakpoint: 1
10   <|b|>print x;
Uncaught exception: Persistent_env.Error(_)

This looks like an error-reporting regression due to #2228, I will investigate -- but this is independent from the bug being reported in the present issue, which is present under both trunk and 4.08.