Since you’re getting the average, it means your network is underfitting the data. Either there aren’t patterns to exploit, or if there are, they’re not found, or at the very least not decoded by the LSTM.
I’d make sure:

bottleneck dimension (LSTM’s hidden state) isn’t too small
data has some patterns (i.e. not white noise)
check that there’s variation in the hidden state, between data samples, by comparing with cosine distance. Use data samples with the same average as a control. If there’s very little variation it means the encoder is underfitting for sure. If there is variation for data with the same average, the decoder is failing somehow.