I believe @TomD8 has the final word on this, if I recall from past discussions.
Firstly, the general rule is that if you are working out of state for your convenience, you owe NY income tax. You would file an NY non-resident return to report your NY-source income, and a MD resident return to report all your worldwide income. (Your other income, like investments, is not considered NY-source and is only reported in MD.) MD will give you a credit for out of state taxes you paid to NY, which reduces the effects of double taxation.
If you are working out of state for the employer's convenience, you do not owe NY state income tax. This might be that the company has no more office space for you, or they want you to be in MD for some business reason of theirs, such as to more easily interact with their MD customers. It depends on the facts and circumstances of each job.
Then second, there is a narrow interpretation of the rule that says that if you never work inside NY, even one day, then NY can't tax you even if you are working remotely for your own convenience. However, if you even work in NY one day a year, such as for training, or a monthly staff meeting that you are required to attend in person, then your job becomes sufficiently connected to NY state that all your remote income is taxable. (This would apply if you work while residing in NY even temporarily, so if you traveled to NY for a work conference that was held at a hotel instead of the company offices, the rule still applies.)
The reason for this exception is that NY can only tax you if your job has a financial connection or nexus to NY. Just working for an NY based employer is not enough, but if you work for an NY employer even one day inside NY borders, then your job for the whole year is "connected" to NY state.
