I found an answer (thanks to David) from Liz Skaates from Salesforce in the Release Community (Oct 8th), talking about what the issue and workaround is, and that they're trying to roll back part of the release. Our support case remains unanswered.
Liz Skaates (Salesforce):
Hi all, apologies for this surprise change in behavior in Winter '19.
Here is a summary of the intent of the change, as well as our plans
for addressing the behaviors you're seeing:
- The intent of the change: We wanted to address the issue of "null" or no role being selected for report types that use role hierarchy
filters. For CRTs, even if you chose to filter by a null role
hierarchy, the default was to filter by the running user's role. We
wanted to make it such that a "null" role selection is the same as
choosing the top of the role hierarchy, which should behaviorally be
the same.
- The actual effects of the change: The change we introduced in Winter '19 did not properly address the scenario of orgs where there are
multiple top-level roles defined, when using "All opportunities" with
the role hierarchy filter. The filter would only be able to use one of
the top-level roles in the filter. This meant that not all
opportunities would actually be returned
- What we're planning to do: We are planning to roll back the Winter '19 change until we can properly support an "all roles" selection for
the role hierarchy filter. This is not an easy roll-back, though, so
it will take some time (potentially a couple weeks).
- Workaround: In the meantime, as we try to roll this back for Winter '19, the workarounds that are (and have already been mentioned in this
thread by others): (a) add a single top-level role manually to your
role hierarchy and move existing roles underneath the new role, (b)
use Classic for now.