very strange bug with LaurentSeriesRing

<div id="comment:0"></div>


```
sage: R.<u> = LaurentSeriesRing(pAdicField(5, 10))
sage: S.<t> = LaurentSeriesRing(RationalField())
sage: R(t + t^2 + O(t^3))
 u^1 + t + O(t^2) + u^2 + t + O(t^2) + O(u^3 + t + O(t^2))
```

???!!!!???


Component: **basic arithmetic**

_Issue created by migration from https://trac.sagemath.org/ticket/7_

