Hello,
Thank you for the report, you are right, this is a regression on our side. On the product edit page, the built-in blocks could still be reordered, but a block that you create yourself did not receive the up/down move buttons and was not registered in the block order, so it stayed at the bottom and lost its position when you saved.
This is now fixed. A patched 6.5.1 build with the correction is available, so please update HikaShop (download the install package on our website in My account>My subscriptions and install it on your website via the Joomla extensions installer) to get it. After updating, a block you create gets its move buttons straight away and keeps the position you give it after saving, with no manual change needed.