OK, I've answered my own question. Apparently dryer sheets will over time leave a coating on the moisture-sensing metal element, making the dryer think that the load is dryer than it really is. The sheets can also leave a coating on the fine mesh of the lint filter. You can wash the lint filter in soapy water. For the moisture-sensing element, the recommendation is to use a cotton swab or cotton ball dipped in rubbing alcohol.

I have seen some posts on various web sites that indicate that LG doesn't honor their warranty on dryers, if they see that dryer sheets have been used. But this may be something that they USED to do. While LG seems to advise against the use of dryer sheets, I don't see any warnings about voiding the warranty in their owner's manual.

In any case, the moisture-sensor metal strip on my new LG dryer is mounted right onto the inner side of the lint filter housing, and would be easily cleaned. For that matter, a new metal element is a cheap part, and seems easily replaced....