MIUI 10 ? Have you check in developer options, last 4 options ?
Frankly speaking I never use this "Force allow apps to external storage" so not sure if this is what you're looking for.

Go to Settings - About phone - click MIUI version 7 times to activate Developer options

Once activated, Settings - Additional settings - you should find it (last 3 menu)

My note 6 also always running out of space even though I have less of my own apps now. Its because Xiaomi keep forcing on us its own apps which keep increasing over time and takes up so much space and they can't be deleted.

Reformat? 😅

My phone still on miui 10.3

Delete all data files - WhatsApp etc

https://miui.blog/any-devices/steps-enable-...miui-11-phones/

10 Steps to Enable Adoptable Storage on MIUI 11 Phones 1
This trick helps you enable the built-in adoptable storage on Xiaomi phones running MIUI 11 without unlocking bootloader nor rooting. This feature allows you to use an external Micro SD Card like internal storage. Hence, you can move apps and their data to the external SD Card.

Previously, we posted a dirty little trick on how to enable adoptable storage using a Windows-based tool. The one you are going to see below does not need any additional tool nor a computer. Therefore, it can be done easier and more efficient.

Once mounted on a specific device, the SD Card cannot be used on other devices. Once the SD Card is adopted by an Android phone, it bounds with that device forever until you format it.

follow the one i share on 1st page. safer data.
it will partition ur sdcard into 2 part, 1 for the apps
another 1 u can use for ur data

i.e use 20-80%
meaning 20% for apps install so if u use 128gb it will be
around 25gb for the apps and the rest u can use for data backup
for pics taken, etc so u wont lose data

using the windows pc method works for me for some time
since i bought 128gb then now i sort of dont need dat workaround anymore