We've noticed recently an odd behaviour that we don't recall seeing prior to v15.0.3. Tested on Mac Sierra with FMA v15.0.3. On Win10 with v15.0.2 this doesn't happen,

When switching from a list layout to a form layout, the form layout that s being loaded is very briefly loaded in the body part of the list layout before the layout is refreshed and loaded normally. We've been able to reproduce this in various older apps (no change to the app).
We've found a temporary solution to prevent the 'flicker' from showing by entering find mode, then switching layout and then going back to browse mode.

Has anyone else encountered this?

old theme perhaps?  Classic?

Nope, custom theme born from enlightened in the app we first noticed it in, a new one being developed from scratch. Other apps we tested briefly were modified enlightened.

Did you try rebuilding the indexes?

Here is a topic that may interest you.

HTH

Lee