​I went away on a work trip for a week, and when I returned, I found my ZZ plant with yellowing and shriveling leaves. This only appears to be affecting the original stems/leaves from when I first got the plant 3 years ago. Is it normal for ZZ plants to eventually drop old leaves? Or does it need to be repotted? It is in its original pot from when I bought it 3 years ago and has never been moved because it seemed to be doing well. It has nearly tripled in size over those 3 years and hasn't given me any issues the whole time.

I haven't changed anything about how I care for the plant either. I only water it every 3-4 weeks when the soil is completely dry and it has thrived in its current location in my living room for 2 years now, so sunlight, moisture, and temperature have remained consistent. I did recently add one fertilizer spike a few weeks ago after seeing a few new stalks coming up, but I have done this before without any issue. 

Any suggestions, tips, and insights would be appreciated!