How to fix the «Notice: Undefined index: 1.0000 in vendor/mirasvit/module-rewards/src/Rewards/Helper/Tier/Rule.php on line 65» bug in the Mirasvit's «Reward Points + Referral program»?

Replace the line in the vendor/mirasvit/module-rewards/src/Rewards/Helper/Tier/Rule.php file:

$currantTier = $customer->getData(TierInterface::CUSTOMER_KEY_TIER_ID);

with the following one:

$currantTier = intval($customer->getData(TierInterface::CUSTOMER_KEY_TIER_ID));

See also:

As I see, the issue is occured in many places, so you should find all occurencies of
->getData(TierInterface::CUSTOMER_KEY_TIER_ID) and apply intval() to the result.