I have a similar problem with my water heater. Here is what I have found. If I go for 24 hours or so without using any hot water, the water that comes out is not as hot as normal. I think what is happening is that the water in the tank has cooled down a little, but not enough to trigger the thermostat to kick in and reheat the tank.
As soon as I turn on the hot water tap, the cold water coming in to the tank triggers the thermostat, so it starts to heat again, but it’s not going to be quick enough for your shower.
If you want a hot shower, and haven’t used the hot water for a while, turn on the hot water tap in your sink about 15-20 minutes before your shower, and let it run for a minute or so.
I get basically the same thing with my 10 year old Gas Hot Water heater too. It basically seems to boil down to the set point for turning the gas on to reheat being well lower than the set point for turning the gas off when heating.
I suspect that means there is some kind of malfunction but in normal usage it isn’t a problem as both temperatures are hot enough for a shower.
I have mine on an economy setting which I believe gives a wider temp range between off an on. I shower in the morning with no use of cold water. But 1 hour later if I have company it is a bit too hot for this.
Doesn’t really help unless you checked the water temp on the dishwasher. It could have easily triggered the heat cycle so your later showers were fine.