2015 Prius II engine randomly won't shut off/poor mpg

Hello everyone hope someone can help. First I had a 2011 PriusII and averaged 50-54 mpg wit he the best being 58.2 worst 47.8(lie-o-meter). I bought this 2015 used with 22000 on the odometer and now have 44000 and have owned it for 16 months. Best 46.6 worst 42.5. I live in same house and have same job and drive the same route as before and in the same way. I've noticed weird things with the engine not shutting off when it gets warm or at a stop with the engine warm and also will not go into ev mode manually when I hit the button. For instance a few days ago my wife had just driven about 40 miles and about 90 degrees outside. 45 min later I go up to the market which is uphill and 3 miles away and is now 93 outside. I noticed on the way that it wasn't going into ev mode(35mph speed limit). I get to the market and stop and motor keeps running and I have nothing on including ac. Hybrid battery showed 2 bars from full. I sat there for 4 minutes and motor was still running and I could hold the pedal to the floor and motor would just rev up and when I released motor still kept running and would not go into ev on its own nor could I put it into ev mode manually. The hybrid battery dropped a bar during this so I switched the display to see what was happening and it didn't show it was doing anything at all. No arrows charging battery etc. it's not the first time it has done this. Sometimes completely normal for instance this morning at 61 degrees outside I get 3/4 of a mile from my house and it kicked into ev itself at 30mph. Anybody dealt with this before? Sorry post is so long just wanted to try and explain everything thoroughly. Also after being in the store for five minutes I come out and start car and it immediately went into ev itself but as soon as I gave it the slightest bit of gas it kicked the motor on and did that the whole way home. Stop and ev mode comes on then the slightest bit of gas and would kick it out. I took a video of it doing this but it won't let me upload it. If someone wants to see it we can figure out a way to get it sent to you. One other thing it has a new optima 12v battery but did this before and after it being changed(my kids got into it and turned on lights etc and killed battery) Thanks in advance for any help!

 

Thanks for the replies. I just looked at the vent before but not up close and appeared clean but just looked inside and saw where that filter is and it's pretty clogged. I thought that it would have to do with hv batteries too warm but wasn't sure. Took to dealership yesterday and got the standard run around. Young kid as service advisor kept saying that there's so many things that have to be perfect to kick into ev and that what was happening to mine is normal. Mechanic said that the video I took was weird but only did a health check and a mpg check where he took it out on the freeway for 10 miles but didn't reset the mpg until actually the freeway. Can't believe they didn't check the filter for the hv battery. Anything else I should look for when I'm in there cleaning the filter? As always thanks for the help and I will keep posted as to whether this fixes the issue!

 

Ok thanks I look up in here how to get to the filter and fan blades. Thanks again everyone!

 

Thanks for the video that's going to help for sure. Here's my filter

 

Will definitely do. Thanks again!!