You can use either Send an email or Send an email (V2) as you'd like. However, they are adding all new improvements to V2 going forward - and eventually, Send an email will likely be deprecated.
As far as your issues are concerned, it is important to remember that Send an email (V2) is currently available in Preview, which means it is still in active development - think of it as being in beta.
Because of this, it won't be perfect for the time being, there will be some minor issues using it. You are also encouraged to provide feedback when using preview items (which you are doing here =).
With that said, as Send an email may eventually be deprecated, I'd strongly recommend that you utilize the Send an email (V2) if and when possible. That way, if the original Send an email is deprecated, you don't need to update your existing Flows to utilize the Send an email (V2).
NOTE: I do not have any information indicating that Send an email will ever be deprecated, however, going on their past improvements to existing connectors, actions and triggers while deprecating older ones - it is certainly within the realm of possibility that it will deprecated at some point in the future. But, it also may never be deprecated.
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