Observed on cswiki since around 2019-10-21 14:40 UTC.

Steps to Reproduce:

Create a lua module with this content:

local p = {}

p.myFunc = mw.wikibase.getEntityIdForCurrentPage

p.myFunc2 = mw.isSubsting

p.getCurrentTitle = mw.title.getCurrentTitle

return p

and then save a page with this content:

{{#invoke:MyModule|myFunc}} {{#invoke:MyModule|myFunc2}} {{#invoke:MyModule|getCurrentTitle}}

Actual Results:
You will observe an error like:
Lua error in mw.lua at line 499: attempt to call field 'getCurrentFrame' (a nil value). (refers to this line of code)

Expected Results:
The output of the module should be the return the return value of those functions, with no error message.

Perhaps this is an undefined behavior or there was a breaking change that might need an announcement.