process_type: update / updater: refresh deletes all trees from existing model

I expect the above code to output a model with 100 trees in both cases. However, running this python script instead gives me:

model size before refresh 100
model size after refresh 10

Which is unexpected. This is on xgboost 2.0.3.

Am I doing something wrong / misunderstanding how the refresh parameter should work?

Reproduce the Python script shown in the issue with XGBoost 2.0.3 and compare the model size before and after refresh. Trace the refresh updater behavior to determine whether retaining all 100 trees is expected, then add a regression check or document the confirmed behavior.

Written by the indexing model from the issue text.