I often have the same issue: a GA "Date + Hour" dimension in a time chart ends up with only a Date granularity. When this happens, one can notice that the dimension tooltip does indicate a "(Date)" suffix:

This tooltip suffix actually gives the hint on how the granularity used by the chart.

To change this, click on the left part of the dimension field to edit it (the little calendar icon turns into a pen icon). From there you can change the type of the field to "Date & Time > Date Hour". This should change the time chart back to the expected "Days + Hours" granularity.

If one then hovers on the dimension field, the tooltip should no longer have the "(Date)" suffix.

For reference, the Looker Studio docs has some more info on changing fields types. There are some limitations apparently.
https://cloud.google.com/looker/docs/studio/dates-and-times