Yes this is your issue, if your ambients are low 60% is not enough. Depending on how low not nearly enough. The heat lamp will increase the air temp and allow the air to carry more water not less. They actually can help humidity not hurt in it is evidenced by the fact the % lowers.
If I ask for a 10% deposit on a given 100$ snake I want 10$ but if you change the snake to a 1000$ snake 10$ is now only 1%. RH is a percent of what you have against what you could have a dropping % only means you have more potential not lower water. (IE you still have 10$ if it is 10% or 1%)