When I put an LTN Combinator, its default setting is provider. I click requester, then close and open the GUI, and it automatically becomes provider.
I set the provider and requester, then close and open the GUI, and it will still automatically become provider.
Only when using LTN Combinator in blueprint can it be requester, but the problem becomes that I can't change it to provider.
This really confuses me, and I remember that it was freely adjustable before. I don't know if it was a bug or if I missed some important operations.

The Provider / Requester setting in the combinator is a pseudo setting. There really isn't an actual state that is set in LTN. Every station can provide and request simultaneously. Negative signals make it request from the network and positive signals provide to the network. The check boxes in the combinator are initialized when you open the GUI based on the signals the combinator is emitting. If it has a negative signal set, it will check the requestor box. There is a mod setting to "High Provide Threshold for Requester". When this setting is 'True' (default) it sets a provider threshold of 50,000,000. If the combinator is emitting this signal, it will uncheck the provider box.

Manually checking these boxed adds / removes the high provider threshold. I think you may have the mod setting "High Provide Threshold for Requester" set to false. That will disable the setting of that signal by the checkboxes and cause the provider/requester boxes to be indicators only.

Alternatively, you may manually set the provide threshold to 50,000,000 to uncheck the 'Provider' box.

Yes, I did turn off the high provide threshold for requester, because I was thinking about making the station function purer. I hope Requester and Provider are completely separated.
Thank you for your explanation, which gives me a more comprehensive understanding of LTN.