I have a DGN file that contains Electrical symbols with Subtype tags. I've noticed that when the model's Drawing Scale is changed the Subtype tags do not automatically change their scale factor. If I manually place tags on those same symbols or place plain text, those items will scale as expected with the Drawing Scale factor.
Defect# 350474 has been filed against this issue.
Workaround:
If you are consistently using only a single Drawing Scale, you can also use the Select By Attributes tool to selectively choose only the Subtype tags and then use the Change Text Attributes tool to change the Font size as needed