I have 4 currency (AED, USD, GBP, SAR) in my store AED is my base and i have 2 store (English and Arabic). All the currencies working properly but when i change AED to SAR or SAR to AED it's not working.
Follow this link maybe you missed any steps
after that also make sure to define currency rates here(maybe your AED and SAR currency rates same)
Store -> currency rate
try to clear cache
php bin/magento cache:clean && php bin/magento cache:flush