Apart from the other answers, also verify that your sales cart rule indexer is set to "update on save" - M2 2.3.5
magento indexer:show-mode salesrule_rule
Sales Rule: Update on Save
If it is on "update on schedule", it will not work/apply on the cart page since that rule record not correctly added ...
In this scenario - because the promotion rule is set to subtotal (ex tax) >= $100 it means that the subtotal calculated after the promotion is taken off and before the tax is added.
Well if you want them to spend atleast $100 and give them $100 off you can't calculate that ex tax because the $100 will be taken off and not leave you with enough to ...
I have found the solution.
I simply refreshed the quote after save cart using:
$quoteId = $quote->getId();
$getQuotebyId = $this->quoteRepository->get($quoteId);
The updated code is:
class SamplesRemoveExtra implements ObserverInterface