It may seem ridiculous to bring up this topic again, but here's my take:

The documentation clearly distinguishes between each type of field and its use case:

https://m2.material.io/components/text-fields#filled-text-field

Filled text field Usage

Filled text fields have more visual emphasis than outlined text fields, making them stand out when surrounded by other content and components.

https://m2.material.io/components/text-fields#outlined-text-field

Outlined text field Usage

Outlined text fields have less visual emphasis than filled text fields. When they appear in places like forms, where many text fields are placed together, their reduced emphasis helps simplify the layout.

Material's documentation clearly distinguishes between each type of field and its use case:

https://m2.material.io/components/text-fields#filled-text-field

Filled text field Usage

Filled text fields have more visual emphasis than outlined text fields, making them stand out when surrounded by other content and components.

https://m2.material.io/components/text-fields#outlined-text-field

Outlined text field Usage

Outlined text fields have less visual emphasis than filled text fields. When they appear in places like forms, where many text fields are placed together, their reduced emphasis helps simplify the layout.